Co-reporter:Carole Brown, Adrian Lita, Yuchuan Tao, Nathan Peek, Mark Crosswhite, Melissa Mileham, J. Krzystek, Randall Achey, Riqiang Fu, Jasleen K. Bindra, Matthew Polinski, Youhong Wang, Lambertus J. van de Burgt, David Jeffcoat, Salvatore Profeta Jr., A. E. Stiegman, and Susannah L. Scott
ACS Catalysis November 3, 2017 Volume 7(Issue 11) pp:7442-7442
Publication Date(Web):September 14, 2017
DOI:10.1021/acscatal.7b02677
The structure and mechanism of the formation of sites which initiate ethylene polymerization in the atomically dispersed Phillips catalyst (Cr/SiO2) are two of the great unsolved mysteries of heterogeneous catalysis. After CO or C2H4 reduction of silica-supported CrVI ions to CrII ions in the precatalyst, exposure to ethylene results in the formation of organoCrIII sites that are capable of initiating polymerization without recourse to an external alkylating cocatalyst. In this work, a Phillips catalyst prepared, via sol–gel chemistry, as a mesoporous, optically transparent monolith was reduced with CO to the spectroscopically determined CrII end point. Ethylene causes rapid reoxidation of these CrII sites to CrIII, even at low temperatures. Solid-state 13C CP-MAS NMR, IR, and Raman spectroscopies reveal that the resulting sites contain a vinyl ligand, described as (≡SiO)2CrIII–CH═CH2 although likely with a higher coordination number, which are capable of initiating polymerization. The formation of these vinyl sites is an incommensurate redox reaction involving one-electron oxidation of CrII via ethylene disproportionation. The accompanying formation of organic radical intermediates and their characteristic reaction products suggest that the key step is homolysis of a Cr–ethyl bond. Plausible pathways for the initiation mechanism are suggested.Keywords: active site; bond homolysis; ethylene polymerization; initiation mechanism; Phillips catalyst; sol−gel;
Co-reporter:Gregory B. Dudley, Ranko Richert and A. E. Stiegman
Chemical Science 2015 vol. 6(Issue 4) pp:2144-2152
Publication Date(Web):16 Jan 2015
DOI:10.1039/C4SC03372H
The use of microwave radiation to drive chemical reactions has become ubiquitous in almost all fields of chemistry. In all of these areas it is principally due to rapid and convenient heating resulting in significantly higher rates of reaction, with other advantages including enhanced product selectivity and control of materials properties. Although microwave heating continues to grow as an enabling technology, fundamental research into the nature of microwave heating has not grown at the same rate. In the case of chemical reactions run in homogeneous solution, particularly synthetic organic reactions, there is considerable controversy over the origins of rate enhancement, with a fundamental question being whether there exist microwave-specific effects, distinct from what can be attained under conventional convective heating, that can accelerate a reaction rate. In this Perspective, we discuss unique aspects of microwave heating of molecules in solution and discuss the origin and nature of microwave-specific effects arising from the process of “selective heating” of reactants in solution. Integral to this discussion is work from the field of dielectric relaxation spectroscopy, which provides a model for selective heating by Debye relaxation processes. The Perspective also includes a critical discussion of hypotheses of non-thermal effects (alternatively classified here as resonant processes) and an outline of specific reaction parameters for chemical systems in which microwave-specific Debye relaxation processes can result in observable reaction rate enhancement.
Co-reporter:Carole Brown, J. Krzystek, Randall Achey, Adrian Lita, Riqiang Fu, Robert W. Meulenberg, Matthew Polinski, Nathan Peek, Youhong Wang, Lambertus J. van de Burgt, Salvatore Profeta Jr., A. E. Stiegman, and Susannah L. Scott
ACS Catalysis 2015 Volume 5(Issue 9) pp:5574
Publication Date(Web):August 13, 2015
DOI:10.1021/acscatal.5b00927
The detailed mechanism by which ethylene polymerization is initiated by the inorganic Phillips catalyst (Cr/SiO2) without recourse to an alkylating cocatalyst remains one of the great unsolved mysteries of heterogeneous catalysis. Generation of the active catalyst starts with reduction of CrVI ions dispersed on silica. A lower oxidation state, generally accepted to be CrII, is required to activate ethylene to form an organoCr active site. In this work, a mesoporous, optically transparent monolith of CrVI/SiO2 was prepared using sol–gel chemistry in order to monitor the reduction process spectroscopically. Using in situ UV–vis spectroscopy, we observed a very clean, stepwise reduction by CO of CrVI first to CrIV, then to CrII. Both the intermediate and final states show XANES consistent with these oxidation state assignments, and aspects of their coordination environments were deduced from Raman and UV–vis spectroscopies. The intermediate CrIV sites are inactive toward ethylene at 80 °C. The CrII sites, which have long been postulated as the end point of CO reduction, were observed directly by high-frequency/high-field EPR spectroscopy. They react quantitatively with ethylene to generate the organoCrIII active sites, characterized by X-ray absorption and UV–vis spectroscopy, which initiate polymerization.Keywords: active site; ethylene polymerization; heterogeneous catalyst; in situ spectroscopy; Phillips catalyst; reaction mechanism; redox activation; sol−gel
Co-reporter:Sharad D. Bhagat;Edmundo Bello Da Silva Filho ;Albert E. Stiegman
Macromolecular Materials and Engineering 2015 Volume 300( Issue 6) pp:580-585
Publication Date(Web):
DOI:10.1002/mame.201500009
Transparent hybrid organic methacrylate capped zirconium oxo-clusters (ZOCs) with thiol-ene monomers. The vinyl capping groups potentially enable the ZOCs to become an integral part of the thiol-ene network. The ZOCs are reasonably soluble in the thiol-ene monomers yielding a loading of 1 wt. -%. The resulting polymer composites were highly transparent in the visible region and exhibited significant enhancement in the refractive indices with measured values ranging from 1. 714 to 1. 744. Moreover, dynamic mechanical analysis (DMA) revealed that all the composites exhibited good mechanical strength and were robust enough to undergo machining and/or polishing without fracture.
Co-reporter:Po-Kai Chen, Michael R. Rosana, Gregory B. Dudley, and A. E. Stiegman
The Journal of Organic Chemistry 2014 Volume 79(Issue 16) pp:7425-7436
Publication Date(Web):July 22, 2014
DOI:10.1021/jo5011526
Under appropriate conditions, significant microwave-specific enhancement of the reaction rate of an organic chemical reaction can be observed. Specifically, the unimolecular Claisen rearrangement of allyl p-nitrophenyl ether (ApNE) dissolved in naphthalene was studied under microwave heating and conventional convective (thermal) heating. Under constant microwave power, reaching a temperature of 185 °C, a 4-fold rate enhancement was observed in the microwave over that using convective heating; this means that the microwave reaction was proceeding at an effective temperature of 202 °C. Conversely, under constant temperature microwave conditions (200 °C), a negligible (∼1.5-fold) microwave-specific rate enhancement was observed. The largest microwave-specific rate enhancement was observed when a series of 300 W pulses, programmed for 145–175 °C and 85–155 °C cycles, where 2- and 9-fold rate enhancements, over what would be predicted by conventional thermal heating, was observed, respectively. The postulated origins of the microwave-specific effect are purely thermal and arise from selective heating of ApNE, a microwave-absorbing reactant in a nonabsorbing solvent. Under these conditions, excess heat is accumulated in the domains around the ApNE solute so that it experiences a higher effective temperature than the measured temperature of the bulk medium, resulting in an accelerated unimolecular rearrangement.
Co-reporter:Anthony Ferrari ; Jacob Hunt ; Adrian Lita ; Bridgett Ashley
The Journal of Physical Chemistry C 2014 Volume 118(Issue 18) pp:9346-9356
Publication Date(Web):April 8, 2014
DOI:10.1021/jp501206n
The steam–carbon reaction, which is the essential reaction of the gasification processes of carbon-based feed stocks (e.g., coal and biomass), produces synthesis gas (H2 + CO), a synthetically flexible, environmentally benign energy source. The reaction is very endothermic, which mandates high temperatures and a large expenditure of energy to drive the reaction. We have found that using microwave irradiation to selectively heat the carbon leads to dramatically different observed thermodynamics for the reaction. From measurement of the equilibrium constants as a function of temperature, the enthalpy of the reaction under microwave radiation was found to become significantly more exothermic, dropping from 144.2 kJ/mol at the median reaction temperature of 880 K to 15.2 kJ/mol under microwave irradiation. The reaction conditions under which the steam–carbon reaction was run, and under which the equilibrium measurements were determined, consisted of three other reactions that came to equilibrium. These reactions were the Boudouard reaction, which is the reaction of CO2 with carbon to form CO; the water–gas shift reaction, where CO and water react to form H2 and CO2; and the carbon–hydrogen reaction, which generates methane from the reaction of H2 with carbon. We determined the equilibrium constants and thermodynamic parameters for all of these reactions. The Boudouard reaction, which is also strongly endothermic, was found to be more exothermic under microwave radiation (180.2 kJ/mol (thermal) and 27.0 kJ/mol (MW)). The water–gas shift reaction became more endothermic (−36.0 kJ/mol (thermal) and −11.4 kJ/mol (MW)). The carbon–hydrogen reaction also underwent an endothermic shift, from −79.7 to −9.1 kJ/mol. From the associated equilibrium expressions and the equilibrium constants for the steam–carbon reaction system, the mole fractions of the system components under thermal and microwave conditions were estimated. The effect of the microwave radiation was to change the position of the equilibrium so that the temperature at which H2 was at a maximum dropped from 643 °C in the conventional thermal reaction to 213 °C in the microwave. Notwithstanding the predicted temperature shift, there was an observable threshold below which microwaves could not produce products. In our system, the minimum energy at which H2 appeared was 373 °C (30 W), while the temperature at which equilibrium could be established in a reasonable period of time (100 min) was 491 °C (100 W).
Co-reporter:Mark Crosswhite, Jacob Hunt, Taylor Southworth, Kyle Serniak, Anthony Ferrari, and A. E. Stiegman
ACS Catalysis 2013 Volume 3(Issue 6) pp:1318
Publication Date(Web):May 6, 2013
DOI:10.1021/cs400109s
A series of heterogeneous catalyst materials possessing good microwave absorption properties were investigated for their activity as oxidation catalysts under microwave irradiation. These catalysts, a series of nanoscale magnetic spinel oxides of the composition MCr2O4 (M = Cu, Co, Fe), were irradiated in aqueous methanol solution (1:1 MeOH:H2O v:v). This resulted in rapid conversion of methanol to formaldehyde, directly generating aqueous formalin solutions. The catalytic reaction occurred under relatively mild conditions (1 atm O2, 60 °C), with irradiation times of 80 min converting 24.5%, 17.7%, and 13.2% of the available methanol to formaldehyde by the Cu, Fe, and Co chromite spinel catalysts, respectively. Importantly, reactions run under identical conditions of concentration, time, and temperature using traditional convective heating yielded dramatically lower amounts of conversions; specifically, 1.0% and 0.21% conversions were observed with Cu and Co spinels, and no observable thermal products were obtained from the Fe spinels. This work provides a clear demonstration that microwave-driven catalysis can yield enhanced reactivity and can afford new catalytic pathways.Keywords: alcohol oxidation; formaldehyde; formalin; methanol oxidation; microwave catalysis; microwave synthesis
Co-reporter:Jacob Hunt ; Anthony Ferrari ; Adrian Lita ; Mark Crosswhite ; Bridgett Ashley
The Journal of Physical Chemistry C 2013 Volume 117(Issue 51) pp:26871-26880
Publication Date(Web):October 30, 2013
DOI:10.1021/jp4076965
The Boudouard reaction, which is the reaction of carbon and carbon dioxide to produce carbon monoxide, represents a simple and straightforward method for the remediation of carbon dioxide in the environment through reduction: CO2(g) + C(s) ⇌ 2CO. However, due to the large positive enthalpy, typically reported to be 172 kJ/mol under standard conditions at 298 K, the equilibrium does not favor CO production until temperatures >700 °C, when the entropic term, −TΔS, begins to dominate and the free energy becomes negative. We have found that, under microwave irradiation to selectively heat the carbon, dramatically different thermodynamics for the reaction are observed. During kinetic studies of the reaction under conditions of flowing CO2, the apparent activation energy dropped from 118.4 kJ/mol under conventional convective heating to 38.5 kJ/mol under microwave irradiation. From measurement of the equilibrium constants as a function of temperature, the enthalpy of the reaction dropped from 183.3 kJ/mol at ∼1100 K to 33.4 kJ/mol at the same temperature under microwave irradiation. This changes the position of the equilibrium so that the temperature at which CO becomes the major product drops from 643 °C in the conventional thermal reaction to 213 °C in the microwave. The observed reduction in the apparent enthalpy of the microwave driven reaction, compared to what is determined for the thermal reaction from standard heats of formation, can be thought of as arising from additional energy being put into the carbon by the microwaves, effectively increasing its apparent standard enthalpy. Mechanistically, it is hypothesized that the enhanced reactivity arises from the interaction of CO2 with the steady-state concentration of electron–hole pairs that are present at the surface of the carbon due to the space-charge mechanism, by which microwaves are known to heat carbon. Such a mechanism is unique to microwave-induced heating and, given the effect it has on the thermodynamics of the Boudouard reaction, suggests that its use may yield energy savings in driving the general class of gas–carbon reactions.
Co-reporter:Yuchuan Tao ; Adrian Lita ; Lambertus J. van de Burgt ; Haidong Zhou
Inorganic Chemistry 2012 Volume 51(Issue 4) pp:2432-2437
Publication Date(Web):January 9, 2012
DOI:10.1021/ic202401a
Cr6+ ions were incorporated into the lattice sites of phase-pure silicalite-2 made using 3,5-dimethylpiperidinium as a structure-directing agent. The materials exhibited a remarkably well-resolved vibronic emission consisting of a high frequency progression of 987 cm–1, which was assigned to the fundamental symmetric stretching mode of the (Si–O−)2Cr(═O)2 group dominated by the terminal Cr═O stretch. A low frequency progression at 214 cm–1, which was assigned to a symmetric O–Cr–O bending mode, was built on each band of the 987 cm–1 progression. Studies of the vibronic structure of the emission spectrum as a function of temperature and Cr ion concentration reveal an abrupt change in the Franck–Condon factor of the emission at 20 K for samples with very low Cr concentrations (0.03 mol %). The change in the Franck–Condon factor is attributed to a temperature-induced structural change in the coordination sphere of the metal ion. This structural change was found to be accompanied by a concomitant structural change in the lattice structure of the silicalite-2. This structural change, as studied by temperature-dependent X-ray diffraction, did not involve a crystallographic phase change but an abrupt decrease in the unit cell volume, caused specifically by a decrease in the c-axis. This structural change was not observed in pure silicalite-2, indicating that it is not intrinsic to the silicalite lattice. Moreover, no similar structural change was observed at higher Cr loading (1 mol %). This suggests that the presence of the Cr ions and the changes in the coordination geometry they undergo at low temperature induced the observed contraction in the silicalite-2 lattice, in effect acting as a thermal switch that decreases the unit cell volume.
Co-reporter:Sharad D. Bhagat, Jhunu Chatterjee, Banghao Chen, and A. E. Stiegman
Macromolecules 2012 Volume 45(Issue 3) pp:1174-1181
Publication Date(Web):January 20, 2012
DOI:10.1021/ma202467a
The self-initiation of the thiol–ene coupling reaction of tetravinyl monomers containing main group elements and trivinyl heterocycles with alkyl and aryl dithiols resulted in the formation of highly cross-linked prepolymer gels which upon final curing at 120 °C yielded hard, monolithic polymeric materials. Because of the presence of highly polarizable main group elements such as Si, Ge, Sn, and S and the relative absence of highly electronegative elements, the resulting polymers exhibited high refractive indices ranging from 1.590 to 1.703 and Abbe numbers between 24.3 and 45.0. All of the polymers were highly transparent over the UV–vis region of the spectrum. Moreover, due to the high cross-linked density achievable in specific compositions, very hard materials capable of being ground and polished could be produced. The range of compositions produced yields important structure–property relationships, indicating the effect of monomer structure on mechanical and optical properties.
Co-reporter:Adrian Lita ; Yuchuan Tao ; Xisai Ma ; Lambertus van de Burgt
Inorganic Chemistry 2011 Volume 50(Issue 21) pp:11184-11191
Publication Date(Web):October 13, 2011
DOI:10.1021/ic2017352
The systematic incorporation of Cr ions into a phase-pure silicalite-2 lattice was accomplished through hydrothermal synthesis using 3,5-dimethylpiperidinium as a templating agent. The Cr ions, after calcination to remove the template, were in the 6+ oxidation state, with their incorporation into the lattice verified by the systematic expansion of the unit cell as a function of Cr loading. The structures of these materials as revealed by electronic spectroscopy and X-ray absorption near-edge spectroscopy (XANES) were consistent with the dioxo structure typically exhibited by Cr6+ in an amorphous silica matrix. These materials were highly luminescent, with the emission spectra showing an unusually well-resolved vibronic structure characteristic of an emissive site with little inhomogeneous broadening. The site was reduced under flowing CO to Cr4+, as characterized by XANES. The reduction of Cr from 6+ to 4+ resulted in unit-cell volumes that are systematically smaller than those observed with Cr6+, even though the ionic radius of Cr4+ is larger. This is attributed to the fact that the Cr6+ site is not a simple metal ion but a significantly larger [CrO2]2+ unit, requiring a larger lattice expansion to accommodate it. Through analysis of the XANES preedge and assignment of the ligand-field spectrum of the Cr4+ ions, it is possible to establish isomorphic substitution into the silicalite lattice.
Co-reporter:Chi-Dong Park, Melissa Mileham, Lambertus J. van de Burgt, Erik A. Muller and A. E. Stiegman
The Journal of Physical Chemistry C 2010 Volume 114(Issue 6) pp:2814-2820
Publication Date(Web):January 27, 2010
DOI:10.1021/jp910274w
The effect of the stoichiometry (i.e., the Al/Fe ratio) and the compressed density of the sample on the dynamics of energy release from a series of Al/Fe2O3 metastable intersitital composites (MIC) were investigated. The reaction was initiated photothermally with a single 8 ns pulse of the 1064 nm fundamental of a Nd:YAG laser. The reaction dynamics were measured by using time-resolved spectroscopy of the light emitted from the igniting and deflagrating material. Two fundamental parameters are measured by this approach: the time to initiation and the duration of the deflagration. The effect of Al particle size on these parameters shows that they attain a minimum value at 100 nm, Al with 120 and 50 nm both having longer initiation and deflagration times. The amount of active Al metal relative to the amount of oxide explains this trend as the Al particle size decreases. As the Al/Fe ratio was varied from stoichiometric (1:1 Al:Fe) to slightly fuel rich (1.5:1) there is a significant decrease in the initiation and deflagration times. When fuel is present in larger excess (2:1) the values of these two parameters begin to increase again. For the range of Al sizes, samples were prepared with different applied pressure to yield samples of increasing density. It was found that the initiation time and deflagration duration decreased with increasing density and reached a minimum value at 50−60% theoretical maximum density. This trend was analyzed in terms of the close packing of the particles using Monte Carlo simulations. From this analysis it was determined that the minimum value corresponded to the point at which the maximum packing fraction was reached.
Co-reporter:Chi-dong Park, Jeremy Walker, Rina Tannenbaum, A. E. Stiegman, J. Frydrych and L. Machala
ACS Applied Materials & Interfaces 2009 Volume 1(Issue 9) pp:1843
Publication Date(Web):August 24, 2009
DOI:10.1021/am900362x
Uniform high-quality iron oxide thin films can be formed from the spin coating of iron oxide/hydroxide sol−gels on a silicon substrate. Thermal processing of the films at temperatures of ∼300 °C results in the transformation of films into a ternary layered structure with iron oxide, Fe2O3, at the surface, characterized by Mössbauer spectroscopy, and reduced, metallic iron characterized by depth profiling of the surface by X-ray photoelectron spectroscopy as a function of Ar+ etching. Imaging of the etched surface by scanning electron microscopy reveals two distinct regions at the interface, nanoparticles that are very iron-rich separated by an unstructured region that is somewhat less iron-rich. The results demonstrate a synthetic protocol for the spontaneous formaton of a ternary layered structure from a simple one-step preparation.Keywords: interfacial redox reactions; iron oxide; iron oxide thin films; thermite reaction
Co-reporter:Cristina Moisii, Lambertus J. van de Burgt and A. E. Stiegman
Chemistry of Materials 2008 Volume 20(Issue 12) pp:3927
Publication Date(Web):May 21, 2008
DOI:10.1021/cm800095g
Vanadium oxide deposited as discrete oxovanadium groups, [(−O)3V═O], in transparent silica xerogels were investigated by resonance Raman spectroscopy. Spectra were collected at 351 and 257 nm excitation into two distinct absorption bands of the oxovanadium site. Three new bands associated with vibrations of the vanadium oxide site were observed at 496, 568, and 720 cm−1. From these additional modes and the previously known vibrations at 1064, 1033, and 923 cm−1 an empirical force field was determined from which a normal-mode analysis of the primary stretching vibrations of the vanadium oxo group was carried out. This analysis indicates that for most of the observed bands the interfacial Si−O−V stretches are the primary component, and in fact, only the weak band at 923 cm−1 was dominated by the terminal V═O stretch. Shifts in the band positions with 18O isotopic enrichment are in general agreement with the normal-mode analysis, moreover, the enrichment indicates that the bridging groups are generally quite labile to substitution.
Co-reporter:Adrian Lita, Xisai Ma, Robert W. Meulenberg, Tony van Buuren and A. E. Stiegman
Inorganic Chemistry 2008 Volume 47(Issue 16) pp:7302-7308
Publication Date(Web):July 12, 2008
DOI:10.1021/ic800366j
Manganese silicalite-2 was synthesized at high pH using the molecular cluster Mn12O12(O2CCH3)16 as a Mn source. The silicalite-2 (ZSM-11) materials were synthesized using 3,5-dimethyl-N,N-diethylpiperdinium hydroxide as a structure-directing agent to produce phase-pure ZSM-11 materials. No precipitation of manganese hydroxide was observed, and synthesis resulted in the incorporation of up to 2.5 mol % Mn into the silicalite-2 with direct substitution into the framework verified by the linear relationship between the unit cell volume and loading. The Mn is reduced to MnII during hydrothermal synthesis and incorporated into the silicalite-2 framework during calcination at 500 °C. Further calcination at 750 °C does not affect the crystallinity but oxidizes essentially all of the MnII to MnIII in the framework. The large difference in oxidation temperatures between the II and III oxidation states provides a means of producing relatively pure manganese(II) and manganese(III) silicalite-2 materials for applications such as catalysis.
Co-reporter:Melissa L. Mileham, Chi-Dong Park, Lambertus J. van de Burgt, Michael P. Kramer and A. E. Stiegman
The Journal of Physical Chemistry A 2008 Volume 112(Issue 49) pp:12568-12571
Publication Date(Web):October 23, 2008
DOI:10.1021/jp8064344
The organic high-energy material pentaerythritol tetranitrate (PETN) was incorporated at low concentrations into Al (100 nm)/Fe2O3 metastable intersitital composites (MIC) to form a hybrid organic/inorganic high-energy material. Studies of the dynamics of energy release were carried out by initiating the reaction photothermally with a single 8 ns pulse of the 1064 nm fundamental of a Nd:YAG laser. The reaction dynamics were measured using time-resolved spectroscopy of the light emitted from the deflagrating material. Two parameters were measured: the time to initiation and the duration of the deflagration. The presence of small amounts of PETN (16 mg/g of MIC) results in a dramatic decrease in the initiation time. This is attributed to a contribution to the temperature of the reacting system from the combustion of the PETN that, at lower loadings, appears to follow an Arrhenius dependence. The presence of PETN was also found to reduce the energy density required for single-pulse photothermal initiation by an order of magnitude, suggesting that hybrid materials such as this may be engineered to optimize their use as an efficient photodetonation medium.
Co-reporter:Joshua G. Moore Dr.;Eric J. Lochner Dr.;Albert E. Stiegman Dr.
Angewandte Chemie International Edition 2007 Volume 46(Issue 45) pp:
Publication Date(Web):5 OCT 2007
DOI:10.1002/anie.200701344
A carbon copy that's in a glass of its own: Nanoparticle structures are formed from the gelation of a silica sol–gel solution and the precipitation of a KIXNiIIY[FeIII(CN)6] Prussian-blue analogue in the sol–gel solution. Calcination of the resulting nanoparticle glass destroys the complex, leaves imprints in the silica, and deposits graphite crystals oriented along the long dimension of the void.
Co-reporter:Joshua G. Moore Dr.;Eric J. Lochner Dr.;Albert E. Stiegman Dr.
Angewandte Chemie 2007 Volume 119(Issue 45) pp:
Publication Date(Web):5 OCT 2007
DOI:10.1002/ange.200701344
Kohlenstoff hinter Glas: Nanopartikelstrukturen entstehen durch Gelieren einer Siliciumoxid-Sol-Gel-Lösung und Abscheidung des Preußischblau-Analogons KIXNiIIY[FeIII(CN)6] in der Sol-Gel-Lösung. Das Kalzinieren des so erhaltenen Nanopartikelglases zerstört den Komplex. Dieser Prozess hinterlässt Hohlräume im Siliciumdioxid, entlang deren Längsseite sich Graphitkristalle abscheiden.
Co-reporter:G. F. Strouse;L. Sorensen;A. E. Stiegman
Advanced Materials 2006 Volume 18(Issue 15) pp:
Publication Date(Web):27 JUL 2006
DOI:10.1002/adma.200690061
Luminescent CdSe quantum dots 2.5 and 6.0 nm in diameter can be incorporated into low-density silica aerogel matrices. In work reported by Stiegmann and co-workers on p. 1965, aerogels are formed from the supercritical CO2 extraction of an alcogel containing quantum dots surface passivated with 3-aminopropyltriethoxysilane. The resulting aerogels, shown on the cover, are low scattering and display intense, stable luminescence.
Co-reporter:L. Sorensen;G. F. Strouse;A. E. Stiegman
Advanced Materials 2006 Volume 18(Issue 15) pp:1965-1967
Publication Date(Web):27 JUL 2006
DOI:10.1002/adma.200600791
Luminescent CdSe quantum dots of 2.5 and 6.0 nm dimension have been incorporated into a low-density silica aerogels matrix. The aerogels are formed from the supercritical CO2 extraction of an alcogel containing quantum dots surface passivated with 3-aminopropyltriethoxysilane. The resulting aerogels (see figure and cover) are low scattering and show intense, stable luminescence.
Co-reporter:Cristina Moisii, Matthew D. Curran, Lambertus J. van de Burgt and A. E. Stiegman
Journal of Materials Chemistry A 2005 vol. 15(Issue 34) pp:3519-3524
Publication Date(Web):28 Jul 2005
DOI:10.1039/B505661F
Homogeneous vanadia–silica xerogels containing vanadium loadings up to 14 mol% were investigated by Raman spectroscopy. The materials are completely amorphous and contain vanadia substituted into the continuous random network of the silica. The optical properties afford Raman spectra with very good signal to noise allowing resolution of the vanadium associated vibrational modes even at low concentrations of vanadium. From the study, assignment of the symmetric interfacial Si–O–V stretch was made at 930 cm−1, and at high concentrations of vanadium the formation of V–O–V linkages was observed at 686 cm−1. Also assigned was a transition associated with a perturbed silica mode at 1070 cm−1.
Co-reporter:Paul R. Giunta;Ry P. Washington Dr.;Tedric D. Campbell;Oliver Steinbock Dr. Dr.
Angewandte Chemie 2004 Volume 116(Issue 12) pp:
Publication Date(Web):9 MAR 2004
DOI:10.1002/ange.200352813
Silicatmonolithe mit geordneten Bereichen mikrometergroßer Säulen werden in Polyacrylamidgel-Templaten gebildet. Die Herstellung der Template wird durch ein externes elektrisches Feld gesteuert. Der Ansatz führt zur Selbstorganisation säulenförmiger Muster im Templat (links; Balken=10μm), die sich durch Imprägnieren mit Tetramethylorthosilicat und anschließendes Calcinieren in Silicatmonolithe (rechts) übertragen lassen.
Co-reporter:Paul R. Giunta;Ry P. Washington Dr.;Tedric D. Campbell;Oliver Steinbock Dr. Dr.
Angewandte Chemie International Edition 2004 Volume 43(Issue 12) pp:
Publication Date(Web):9 MAR 2004
DOI:10.1002/anie.200352813
Silica monoliths with ordered arrays of micrometer-scale columns were fabricated from polyacrylamide gel templates. The preparation of the template involves the charged comonomer, N-acryloyl glycine, and is controlled by an externally applied electric field. This approach gives rise to the self-organization of columnar density patterns in the template (left; bar=10μm) that are recovered in the silica monoliths upon impregnation with tetramethylorthosilicate and subsequent calcination (right).
Co-reporter:Joshua G. Moore;Eric J. Lochner;Chris Ramsey;Naresh S. Dalal
Angewandte Chemie International Edition 2003 Volume 42(Issue 24) pp:
Publication Date(Web):17 JUN 2003
DOI:10.1002/anie.200250409
Moldable magnetism: Tunable photomagnetism is exhibited by homogeneous, optically transparent, moldable silica xerogel nanocomposites containing the ferromagnetic Prussian blue analogue KxCo[FeIII(CN)6]z. These materials also show a blue-shifted intervalence charge-transfer band and superparamagnetic behavior. The picture shows the temperature dependence of the field-cooled (FC) and zero-field-cooled (ZFC) dc magnetization μ of a xerogel sample (inset).
Co-reporter:Joshua G. Moore;Eric J. Lochner;Chris Ramsey;Naresh S. Dalal
Angewandte Chemie 2003 Volume 115(Issue 24) pp:
Publication Date(Web):17 JUN 2003
DOI:10.1002/ange.200250409
Formbare Magnete: Homogene Siliciumdioxid-Xerogel-Nanokomposite mit dem Preußisch-Blau-Analogon KxCo[FeIII(CN)6]z sind formbar und optisch transparent, die Intervalenz-Charge-Transfer-Absorptionen sind blauverschoben. Die Proben zeigen abstimmbaren Photomagnetismus und superparamagnetisches Verhalten. Im Diagramm ist die Temperaturabhängigkeit der Magnetisierung μ einer Xerogel-Probe bei Abkühlung mit (FC) und ohne Magnetfeld (ZFC) zu sehen.
Co-reporter:Gregory B. Dudley, Ranko Richert and A. E. Stiegman
Chemical Science (2010-Present) 2015 - vol. 6(Issue 4) pp:NaN2152-2152
Publication Date(Web):2015/01/16
DOI:10.1039/C4SC03372H
The use of microwave radiation to drive chemical reactions has become ubiquitous in almost all fields of chemistry. In all of these areas it is principally due to rapid and convenient heating resulting in significantly higher rates of reaction, with other advantages including enhanced product selectivity and control of materials properties. Although microwave heating continues to grow as an enabling technology, fundamental research into the nature of microwave heating has not grown at the same rate. In the case of chemical reactions run in homogeneous solution, particularly synthetic organic reactions, there is considerable controversy over the origins of rate enhancement, with a fundamental question being whether there exist microwave-specific effects, distinct from what can be attained under conventional convective heating, that can accelerate a reaction rate. In this Perspective, we discuss unique aspects of microwave heating of molecules in solution and discuss the origin and nature of microwave-specific effects arising from the process of “selective heating” of reactants in solution. Integral to this discussion is work from the field of dielectric relaxation spectroscopy, which provides a model for selective heating by Debye relaxation processes. The Perspective also includes a critical discussion of hypotheses of non-thermal effects (alternatively classified here as resonant processes) and an outline of specific reaction parameters for chemical systems in which microwave-specific Debye relaxation processes can result in observable reaction rate enhancement.
Co-reporter:Yu Wu, Josh Gagnier, Gregory B. Dudley and A. E. Stiegman
Chemical Communications 2016 - vol. 52(Issue 75) pp:NaN11283-11283
Publication Date(Web):2016/08/25
DOI:10.1039/C6CC06032C
Chemically reactive molecules that are poor absorbers of microwave radiation can be selectively heated by the microwave and can experience chemical-rate enhancement if they are associated (agglomerated) in solution with non-reactive polar molecules that are strong microwave absorbers.
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